1. Field of the Invention
This invention in general relates to filters for modifying the spectral composition of radiant energy and in particular to a filter which preferably utilizes liquid crystals and polarizers to vary the content of radiation in the visible region of the spectrum to provide any one of a predetermined number of colors.
2. Description of the Prior Art
Color filters are well-known in the photographic and optical arts and include devices and materials which deliberately change the spectral intensity distribution and/or state of polarization of electromagnetic radiation incident upon them for purposes of synthesizing color(s).
One commonly known class of filter, called absorption filters, utilizes chemical dyes dissolved in suitable media such as gelatin or plastic to selectively absorb or remove certain wavelengths of incident radiation, either partially or completely, such that the remaining radiation transmitted by the filter is a desired color. Other known filters utilize thin coatings or films of various substances to create interference effects to synthesize color and are thus referred to as interference filters. Dichroic filters also are well-known being those which selectively absorb radiation polarized in one direction more strongly than that polarized in other directions.
Also well-known are materials, such as ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP), which become optically active in the presence of external magnetic and electric fields. Such materials have been variously combined with plane polarizers and dichroic polarizing filters to provide multicolor filter devices which are capable of producing more than one color in response to an applied electrical or magnetic field.
In the patent literature a number of examples of multicolor filter devices exist. These include U.S. Pat. No. 2,493,200 issued to E. H. Land on Jan. 3, 1950 and entitled "Variable Polarizing Color Filter". The Land patent discloses a color filter which combines a plurality of dichroic polarizers, a plane polarizer, and a plurality of Kerr cells comprising ADP which are arranged to operate as fractional wave retardation plates.
Another example is U.S. Pat. No. 4,068,926 issued to Kenji Nakamura on Jan. 17, 1978 where there is disclosed a twisted nematic liquid crystal cell sandwiched between an achromatic linear polarizer and a colored linear polarizer. This combination is arranged to display colored patterns having light-on-colored or colored-on-light image-background combinations.
A further example of the use of the properties of ADP materials operating as fractional wave retardation plates in combination with polarizers is described in U.S. Pat. No. 2,834,254 issued to Stanley J. Sage on May 13, 1958 and entitled "Electronic Color Filter".
A mechanically operable variable color filter comprising composite chromatic polarizing components, an acromatic polarizing filter, and a quarter-wave retardation plate is described in U.S. Pat. No. 3,936,147 issued to Sanjiro Murakami on Feb. 3, 1976 and entitled "Variable Characteristic Light Filter".
In U.S. Pat. No. 3,994,565 issued to Van Doorn et al. on Nov. 30, 1976 and entitled "Device For Displaying Characters Having A Twisted Nematic Liquid Crystal", two sources of colored light, linearly polarized in different planes, are combined with a liquid crystal having a twisted nematic configuration to display characters on differently colored backgrounds.
A preferred arrangement for a dichroic polarizer, a liquid crystal, a plane polarizer, and a mirror which are configured as a colored display is described in U.S. Pat. No. 4,025,164 issued to Doriguzzi et al. on May 24, 1977 and entitled "Liquid Crystal Display Device For Colored Display Of Information With A Selective Polarizer".
In U.S. Pat. No. 2,586,635 issued to G. L. Fernside on Feb. 19, 1952 and entitled "Color Control Systems", there is described a color filter comprising a combination of a plane polarizer, polarized color filters and birefringent polarization rotators.
And in an article entitled "New Multicolor Liquid Crystal Displays That Use A Twisted Nematic Electro-optical Cell" by Terry J. Scheffer, which appeared in the J. Applied Physics, vol. 44, no. 11, in November 1973, there is described two- and four-color displays which utilize polarizers, liquid crystals, and optical retardation plates.
As evidenced by the aforementioned examples of multicolor filter devices, it is clear that the cynthesis of color is of fundamental importance in the optical and photographic arts. Because of this, it is a primary object of the present invention to provide a color filter for varying the spectral composition of radiant energy to produce a predetermined number of colors.
It is another object of the present invention to provide a color filter for continuously varying the spectral composition of radiant energy to provide any one of a gamut of colored light.
It is another object of the present invention to provide an electro-optical color filter by which any one of a gamut of colors can be produced.
It is yet another object of the present invention to provide a color filter having improved color saturation and hue control through the use of a preferred arrangement of plane polarizers, dichroic polarizers, and polarization rotators.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, the combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure.